Geophys. J. Int. (2008) 174, 1153–1173
doi: 10.1111/j.1365-246X.2008.03860.x
Seamount volcanism along the Gakkel Ridge, Arctic Ocean
James R. Cochran
Lamont Doherty Earth Observatory of Columbia University, Palisades, New York, USA. E-mail: [email protected]
SUMMARY
The Gakkel Ridge in the Arctic Ocean is the slowest spreading portion of the global midocean ridge system. Total spreading rates vary from 12.8 mm yr−1 near Greenland to
6.5 mm yr−1 at the Siberian margin. Melting models predict a dramatic decrease in magma
production and resulting crustal thickness at these low spreading rates. At slow spreading
ridges, small volcanic seamounts are a dominant morphologic feature of the rift valley floor
and an important mechanism in building the oceanic crust. This study quantitatively investigates the extent, nature and distribution of seamount volcanism at the ultraslow Gakkel Ridge,
the manner in which it varies along the ridge axis and the relationship of the volcanoes to
the larger scale rift morphology. A numerical algorithm is used to identify and characterize
isolated volcanic edifices by searching gridded swath-bathymetry data for closed concentric
contours protruding above the surrounding seafloor. A maximum likelihood model is used to
estimate the total number of seamounts and the characteristic height within different seamount
populations. Both the number and size of constructional volcanic features is greatly reduced
at the Gakkel Ridge compared with the Mid-Atlantic Ridge (MAR). The density of seamounts
(number/area) on the rift valley floor of the Western Volcanic Zone (WVZ) is ∼55% that of
the MAR. The observed volcanoes are also much smaller, so, the amount of erupted material
is greatly reduced compared with the MAR. However, the WVZ is still able to maintain a
MAR-like morphology with axial volcanic ridges, volcanoes scattered on the valley floor and
rift valley walls consisting of high-angle faults. Seamount density at the Eastern Volcanic
Zone (EVZ) is ∼45% that of the WVZ (∼25% that of the MAR). Seamounts are clustered
at the widely spaced magmatic centres characteristic of the EVZ, although some seamounts
are found between magmatic centres. These seamounts tend to be located at the edge of the
rift valley or on the valley walls rather than on the valley floor. Seamounts in the Sparsely
Magmatic Zone (SMZ) are located almost entirely at the 19◦ E magmatic centre with none
observed within a 185 km-long portion of the rift valley floor. The EVZ and SMZ appear to
display a mode of crustal accretion, characterized by extreme focusing of melt to the magmatic
centres. Magmas erupted between the magmatic centres appear to have ascended along faults.
This is very different from what is observed at the WVZ (or the MAR), and there is a threshold
transition between the two modes of crustal accretion. At the Gakkel Ridge, the location of
the transition appears to be localized by a boundary in mantle composition.
Key words: Mid-ocean ridge processes; Effusive volcanism; Eruption mechanisms and flow
emplacement; Arctic region.
I N T RO D U C T I O N
The Gakkel Ridge, the spreading centre in the Eurasian Basin of
the Arctic Ocean (Fig. 1), is the slowest spreading section of the
global mid-ocean ridge system. Total spreading rates vary from
12.8 mm yr−1 at 83◦ N, 6◦ W near the intersection of the Gakkel
Ridge and the Lena Trough to 6.5 mm yr−1 at 78.5◦ N, 128◦ E near
the Siberian continental margin (Karasik 1968; Vogt et al. 1979;
DeMets et al. 1994; Brozena et al. 2003). For comparison, spreading
rates along the Southwest Indian Ridge (SWIR), the other ‘ultraslow’ spreading ridge, vary between about 16 and 12 mm yr−1
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Gordon 1999). Despite its extremely low spreading rate, a welldefined, localized ridge axis, identifiable through bathymetry, seismic reflection and potential field data (Vogt et al. 1979; Sekretov
1998, 2002; Cochran et al. 2003) and flanked by lineated ridgeparallel magnetic anomalies interpreted as a seafloor spreading
anomaly sequence (e.g. Karasik 1968; Vogt et al. 1979; Glebovsky
et al. 2000, 2006; Brozena et al. 2003), is present along nearly the
entire length of the Gakkel Ridge. Organized seafloor spreading
appears to be occurring even at the very slow spreading rates found
at the Gakkel Ridge.
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GJI Volcanology, geothermics, fluids and rocks
Accepted 2008 May 21. Received 2008 May 21; in original form 2007 June 8
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J. R. Cochran
Figure 1. Bathymetric map showing the location of the Gakkel Ridge within the Eurasian Basin of the Arctic Ocean. Other bathymetric features identified
include the Lena Trough (LR), Lomonosov Ridge (LR) and Knipovich Ridge (KR).
Melting models based on decompression melting of mantle,
which is upwelling in response to corner flow driven by plate separation, predict that melt production, and thus crustal thickness,
should decrease significantly at total spreading rates below ∼15–
20 mm yr−1 (e.g. Reid & Jackson 1981; Bown & White 1994). The
limited amount of seismic refraction and other geophysical data
available from ultra-slow spreading ridges supports this prediction.
Klingelhofer et al. (2000) obtained a mean crustal thickness of
∼4 km from an ocean bottom seismometer (OBS) experiment near
72◦ N on Mohns Ridge, where the spreading rate is 15.8 mm yr−1
(14 mm yr−1 opening perpendicular to the axis). Also Minshull
et al. (2006) determined an average crustal thickness of 4.2 km
from a set of seismic refraction experiments at the SWIR axis near
66◦ E, where the observed spreading rate is in the range of 13.5–
15 mm yr−1 (Cannat et al. 2003). In both cases, a very thin layer
3 is the main difference from ‘normal’ 6–7 km-thick oceanic crust
(e.g. White et al. 1992).
Coakley & Cochran (1998) concluded on the basis of gravity data
that crust at the axis of the Gakkel Ridge has to be less, and perhaps
much less, than 4 km thick. Jokat et al. (2003) and Jokat & SchmidtAursch (2007) reported on 12 seismic refraction experiments in the
Gakkel rift valley between 5◦ W and 67◦ E, carried out using receivers
placed on the ice. Spreading rates in this portion of the Gakkel Ridge
are between 12.7 and 10.5 mm yr−1 . They consistently found a thin
crust (1.2–4.9 km thick), characterized by relatively low seismic
velocities, generally < 6.0 km s−1 . This low velocity crust rests
directly on a layer with compressional velocities of 7.4–7.8 km s−1 ,
interpreted as mantle. The low mantle velocities probably reflect
serpentinization. Seismic velocities typical of oceanic layer 3 were
not found (Jokat & Schmidt-Aursch 2007). It thus appears that the
crust at the Gakkel Ridge may consist largely of basalts erupted
onto peridotite. This is supported by the observation that gabbro
was only rarely recovered during an extensive dredging programme
in the Gakkel rift valley (Michael et al. 2003).
The greatly decreased melt production at the Gakkel Ridge can
be expected to affect not only the crustal thickness and structure
but also the style of volcanic activity at the ridge axis. In this study,
we use multibeam bathymetric data to quantitatively investigate
the form and distribution of volcanic edifices along the Gakkel
Ridge and their relationship to the morphology of the ridge axis. A
numerical algorithm (Behn et al. 2004) is used to identify isolated
volcanic features and determine the location, height, volume and
ellipticity of each. A maximum likelihood model (Smith & Jordan
1988) is used to characterize the population in a given area by
determining the characteristic height and density (seamounts per
1000 km2 ) of the seamount population within a given region.
B AT H Y M E T RY D ATA F R O M
THE GAKKEL RIDGE
The Gakkel Ridge has now been surveyed by two major expeditions
using very different platforms and instrumentation. The SCience
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ICe EXercises (SCICEX) programme utilized U.S. Navy nuclear
submarines for a series of unclassified scientific research cruises
to the Arctic Ocean between 1993 and 1999. In particular, ‘USS
Hawkbill’ carried out a systematic survey of the Gakkel Ridge
axis utilizing the SCAMP instrument package during the 1998 and
1999 cruises. These cruises obtained nearly complete bathymetry,
sidescan and gravity coverage to 50 km from the ridge axis, for
600 km along the Gakkel Ridge, from 8◦ E to 73◦ E, with coverage of the axis to 96◦ E (Edwards et al. 2001; Cochran et al.
2003). The SCAMP bathymetric system (Chayes et al. 1996, 1997,
1999) is an interferometric instrument that produces extremely
high resolution sidescan but only moderate resolution bathymetry.
In addition, due to short period navigational noise, the data had
to be low-pass filtered. As a result, features wider than a few
km across are well imaged, but the small seamounts, which are
the object of this study, have basically been removed from the
data.
The Arctic Mid-Ocean Ridge Expedition (AMORE) programme
utilized two icebreakers (‘USCGC Healy’ and ‘PFS Polarstern’) to
carry out a swath-mapping, rock sampling and seismic refraction
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programme from 7◦ W to 86◦ E in 2001 (Edmonds et al. 2003; Jokat
et al. 2003; Michael et al. 2003). Mapping was carried out with
a Hydrosweep DS2 system on ‘Polarstern’ and a SeaBeam 2112
system on ‘Healy’. Thiede (2002) estimated an absolute accuracy
of about 0.5% of the water depth for both systems. The data are
gridded into 50 m × 50 m cells, which is slightly less than the beam
footprint. Thus, features wider than a few hundred metres can be
resolved. These bathymetry data are of much higher resolution than
the SCICEX data, but since AMORE was largely a rock-sampling
programme, it did not carry out a systematic bathymetric survey
and data are limited to the rift valley floor and portions of the walls,
only rarely extending out of the rift valley.
The two data sets complement each other. The AMORE data
provide high-resolution bathymetric data from the rift valley floor
and portions of the walls (Figs 2, 3a and 4a). It serves as the database
for our effort to identify and characterize volcanic edifices. The
SCICEX data have much lower resolution, but systematically maps
a broad region extending out of the rift valley onto the ridge flanks
(Figs 3b and 4b). We utilize it to establish the tectonic context of
the detailed AMORE data and to examine the relationship between
Figure 2. Bathymetry map of the Western Volcanic Zone (WVZ) based on AMORE data (Michael et al. 2003). Red boxes outline areas searched for seamounts.
White dots show the location of seamounts with height >50 m, identified on the rift valley floor and yellow dots show the location of seamounts on the valley
walls and flanks. Dashed black line shows the boundary between the WVZ and SMZ. Blue box outlines the region shown in Fig. 5a. Map is contoured at
250 m intervals.
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Figure 3. (a) Bathymetry map of the Sparsely Magmatic Zone (SMZ) based on AMORE data (Michael et al. 2003). Red boxes outline areas searched for
seamounts. White dots show the location of seamounts identified with height >50 m on the rift valley floor and yellow dots show the location of seamounts on
the valley walls and flanks. Dashed black lines show along-axis boundaries of the SMZ. Blue box outlines the region shown in Fig. 6a. Map is contoured at
250 m intervals. (b) Bathymetry map of the Sparsely Magmatic Zone (SMZ) based on SCICEX data (Cochran et al. 2003). Solid black lines show submarine
tracks for profiles shown in Fig. 8. Dashed black line shows boundary between the SMZ and EVZ. Map is contoured at 250 m intervals.
the nature of volcanism in the rift valley and the structure of the rift
valley and rift mountains.
G A K K E L R I D G E A X I A L B AT H Y M E T RY
Michael et al. (2003) divided the Gakkel Ridge into three sections
on the basis of bathymetry and rock recovery:
Western Volcanic Zone
The ‘Western Volcanic Zone’ (WVZ) extends about 220 km from
the western end of the ridge near 83◦ N, 7◦ W to 84◦ 30 N, 3◦ E (Fig.
2). This region is characterized by a relatively shallow (∼4200 m)
rift valley, abundant recovery of basalt and numerous volcanic landforms (Michael et al. 2003). A series of bathymetric highs are located on the rift valley floor (Fig. 2). The two most prominent,
centred near 83◦ 24 N, 4◦ 45 W and 83◦ 52 N, 1◦ 45 W, are elongate
volcanic ridges extending for ∼40–50 km along the axis (Figs 2 and
5a). Two other bathymetric highs, near 83◦ 05 N, 6◦ W and 83◦ 37 N,
3◦ 15 W are less elongate and extend for 15–30 km along-axis. A
broader, less well-defined high is located in the north between about
84◦ 12 N and 84◦ 25 N. There is no axial volcanic ridge there, but
nearly circular volcanic cones, a few kilometres across and up to
100–150 m high, are prominent features of the rift valley floor in
this area.
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Figure 4. (a) Bathymetry map of the Eastern Volcanic Zone (EVZ) based on AMORE data (Michael et al. 2003). Red boxes outline areas searched for seamounts.
White dots show the location of seamounts identified with height >50 m on the rift valley floor and yellow dots show the location of seamounts on the valley walls
and flanks. Dashed black line shows the boundary between the EVZ and SMZ. Blue box outlines the region shown in Fig. 9a. Map is contoured at 250 m intervals.
(b) Bathymetry map of the Eastern Volcanic Zone (EVZ) based on SCICEX data (Cochran et al. 2003). Solid black lines show submarine tracks for profiles
shown in Fig. 8. Dashed black line shows boundary between the EVZ and SMZ. Map is contoured at 250 m intervals.
The rift valley narrows slightly at all of the bathymetric highs
and the rift valley floor shallows by 500–1000 m across its entire
width in a manner reminiscent of the intrasegment bathymetric
highs along the Mid-Atlantic Ridge (MAR; e.g., Sempere et al.
1990, 1993; Detrick et al. 1995). Although these highs display
the magmatic segmentation of the ridge, they are not separated by
observable tectonic ridge axis discontinuities, suggesting that the
segmentation is controlled by magmatic processes in the mantle
rather than imposed by lithospheric tectonics (Michael et al. 2003).
The rift valley walls consist of sets of inward facing faults
(Figs 5a and b) that form a pattern of scarps and terraces similar to those observed at the MAR (e.g. Needham & Francheteau
1974; Macdonald 1986). The size of individual scarps tends to become larger to the northeast of 83◦ 55 N, giving the walls a steeper
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scarp heights occasionally reach 1000 m (e.g. 83◦ 59 N-84◦ 03 N,
Figs 5a and b).
Sparsely Magmatic Zone
The ‘Sparsely Magmatic Zone’ (SMZ) extends for about 275 km
from near 84◦ 38 N, 3◦ E to 86◦ N, 29◦ E (Figs 3a and b). The SMZ
is separated from the WVZ by a 10 km left-stepping non-transform
discontinuity. The eastern end of the SMZ is marked by an oblique
spreading axis extending nearly east–west along 86◦ N for 25–
30 km from 25◦ E to 29◦ E (Figs 3a and b).
The rift valley floor deepens rapidly east of the 3◦ E non-transform
offset, reaching 5400 m within 5 km of the offset. The valley floor
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Figure 5. (a) Bathymetry map of the Gakkel Ridge axis from 83◦ 40 N to 84◦ 10 N based on AMORE data (Michael et al. 2003). Location of the area within
the WVZ is shown in Fig. 2. Map is contoured at 100 m intervals and is illuminated from the northwest. White circles show location of seamounts with height
>50 m. Red box outlines the region shown in Fig. 5b. (b) Detailed bathymetry map of a portion of the Gakkel Rift valley within the WVZ. Location is shown
in (a). Map is contoured at 20 m intervals and is illuminated from the northwest. White circles show the location of seamounts with height >50 m identified in
the region.
generally remains very deep (4700–5400 m) throughout the SMZ
(Figs 3a and b), with no systematic pattern of depth variations. The
valley floor is very narrow (< ∼5–8 km across) and is generally
either flat or concave downward with very few obviously volcanic
features (Figs 6a and b). The only areas of volcanic activity identified
within the SMZ are near 13◦ E and 19◦ E, where Michael et al.
(2003) recovered basaltic rocks and which are the only areas of
bright returns within the EVZ in the SCICEX sidescan data (Fig. 7,
Cochran et al. 2003).
Michael et al. (2003) refer to an elongate ridge extending for
∼20 km along the axis and reaching a depth of 4000 m (Figs 3a and
b, 8—profile A) as a volcanic edifice. Unlike the volcanic centres of
the WVZ, it is not associated with a shallowing of the rift valley. It
sits on the deep rift valley floor, which remains at a depth of 5000 m
at the location of the ridge (Fig. 8—profile A). Cochran et al. (2003)
interpreted this ridge as a tectonic rather than a volcanic feature,
based on morphology. They attribute the high backscatter in the
region to flows from two rounded bathymetric highs on the north
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Figure 6. (a) Bathymetry map of the Gakkel Ridge axis from 3◦ 30 E to 11◦ 30 E based on AMORE data (Michael et al. 2003). Location of the area within the
SMZ is shown in Fig. 3a. Map is contoured at 100 m intervals and is illuminated from the northwest. White circles show location of seamounts with height
>50 m. Red box outlines the region shown in (b). Note the lack of seamounts on the valley floor. (b) Detailed bathymetry map of a portion of the Gakkel rift
valley within the SMZ. Location is shown in (a). Map is contoured at 20 m intervals and is illuminated from the northwest. White circles show the location of
seamounts with height >50 m identified in the region. All of the seamounts are located on the rift valley wall and flanks. Some of these features, such as those
near 85◦ 01 N, 6◦ 45 E and 85◦ 03.8 N, 8◦ 08 E are probably of tectonic origin, but others appear to be volcanic.
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Figure 7. Side scan data from the Gakkel Ridge obtained by USS Hawkbill during the 1998 and 1999 SCICEX cruises (Cochran et al. 2003). Black represents
very high-energy returns and white represents very low-energy returns. High-energy, or ‘bright’, returns generally result from the presence of steep slopes or
recent volcanic features with little or no sediment cover. Red arrows show the location of volcanic centres within the rift valley. Heavy black line marks the
SMZ/EVZ boundary.
flank of the rift valley, which they interpret as volcanoes, although
Michael et al. (2003) recovered peridotite from the rift valley wall
below one of them.
The 19◦ E volcanic centre is very different. This feature is not a
volcanic ridge, but rather a shallowing of the entire rift valley to
about 3500 m (Fig. 8—profile D). It is also the site of a prominent
across-axis ridge (Fig. 3b), which gravity data imply is associated
with 2–3 km of crustal thickening (Cochran et al. 2003). Cochran
et al. (2003) suggested that this might be the only location in their
survey area (east of 8◦ E) where ‘typical’ oceanic crust, including
gabbroic layer 3 has been generated. All rock samples from the
19◦ E area are basaltic (Michael et al. 2003).
Minimum depths in the rift mountains are generally in the range
of 2700–3300 m throughout the SMZ, so, rift valley relief is usually
in the range of 1500–2200 m (Figs 3b and 8—profiles A–F). Michael
et al. (2003) note that the transition between the WVZ and SMZ
near 3◦ E is accompanied by a change in the nature of the rift valley
walls from sets of high angle faults in the west to large lower angle
faults to the east. The slope of the first major scarp south of the
axis on six SCICEX lines between 12◦ 40 E and 18◦ E, measuring
the portion of the wall that appears to be a single fault surface
at the resolution of the data, ranges from 13◦ to 19◦ . The vertical
relief on these individual scarps is from 700 to 2300 m (Fig. 8—
profiles A–C).
SCICEX data show that the rift mountains are asymmetric to
the west of the 19◦ N volcanic centre. The southern flank consists
of a series of high, narrow linear ridges separated by troughs that
often are deeper than 4000 m, whereas the north flank has a more
massive, blocky structure with less relief (Fig. 8—profiles A–C).
Such asymmetry is not apparent east of 19◦ E. Lineated ridges are
still present there (Fig. 3b) but are not separated by deep troughs
giving the rift mountains a blockier appearance (Fig. 8—profiles
E, F).
Eastern Volcanic Zone
The ‘Eastern Volcanic Zone’ (EVZ) extends for at least 580 km
from about 29◦ E eastward as far as the axis has been surveyed
(∼96◦ E). This study will be limited to the portion of the EVZ from
29◦ E to 73◦ E, where the rift valley is not inundated with sediments
and we have good bathymetric coverage (Figs 4a and b).
Although referred to as a ‘volcanic zone’ by Michael et al. (2003),
the EVZ is very different from the MAR-like WVZ. The EVZ is
characterized by a very deep (∼ 5000 m) rift valley. A series of
large volcanic structures are spaced along the rift valley floor at
31◦ E, 37◦ E, 43◦ E, 55◦ E and 69◦ E within the well-surveyed portion
of the axis (Figs 4a and b). The rift valley is nearly buried by
sediments east of the large 69◦ E volcanic centre (Fig. 8—profile P).
However, at least two more volcanic centres have been identified
protruding through the sediments near 85◦ E and 93◦ E (Edwards
et al. 2001) and SCICEX sidescan data suggest the presence of
at least two more centres between 69◦ E and 85◦ E (M. Edwards,
private communication, 2007; also see Fig. 7). The volcanic centres
typically extend for about 30 km along the axis and reach depths of
less than 3500 m (e.g. Fig. 9a). The five volcanic centres to the west
of 69◦ E are each associated with a 20–40 mGal mantle Bouguer
(MBA) gravity low (Cochran et al. 2003).
A major difference between the EVZ and the SMZ is that SCICEX sidescan data (Fig. 7) shows bright returns, suggesting recent
volcanism, along the entire length of the rift valley floor from 29◦ E
to 63◦ E (Cochran et al. 2003). This observation holds even through
the ∼85 km stretch of very deep (5000 m), nearly flat rift valley between the 43◦ E and 55◦ E centres. This inference is consistent with
the observation that all but one dredge from the EVZ recovered
basalt, whether the dredges were located on the volcanic highs or in
the deep regions between them (Michael et al. 2003).
The ridge flanks in the EVZ are generally at a depth of 3000–
3500 m with individual ridges having a relief of 500–700 m
(Figs 4b and 8—profiles G–P). Although total relief from the rift
valley floor to the rift mountains is often 1500–2000 m, the largeoffset low angle faults characteristic of the western portion of the
SMZ (Fig. 3b and 8—profiles A–C) are not observed in the EVZ.
Distinct changes in the trend of the ridge axis occur near 30◦ E
and 62◦ E. At each location, a high-standing ridge extends away
from the axis on the ‘inside corner’ (Figs 4b and 8—profiles G, M).
Langseth Ridge, the inside corner ridge at 62◦ E, reaches to within
800 m of the sea surface. These ridges do not have MBA gravity
lows and are not areas of thickened crust (Cochran et al. 2003).
In addition, only altered diabase was recovered from the flanks of
these ridges (Michael et al. 2003). Both Cochran et al. (2003) and
Michael et al. (2003) suggest that these two ridges are primarily of
tectonic rather than volcanic origin.
METHODS
The objective of the bathymetry analysis is to identify and characterize volcanic activity along the Gakkel Ridge in a quantitative
and objective manner. We wish to determine how the number, form,
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Figure 8. (a) Bathymetry and free-water gravity profiles across the Sparsely Magmatic Zone. Profiles are projected along the local spreading direction with
the axis as the origin. North is to the left-hand side (negative distances). The longitude at which each profile crosses the axis is noted. Letters on each profile
correspond to those on Fig. 3b, which shows the locations of the profiles. Red arrow identifies the 13◦ E volcanic centre of Michael et al. (2003). Blue arrows
identify the high, narrow ridges bounded by low angle faults, which are analogous to the ‘smooth terrain’ described by Cannat et al. (2006) on the SWIR.
(b) Bathymetry and free-water gravity profiles across the Eastern Volcanic Zone. Profiles are projected along the local spreading direction with the axis as the
origin. North is to the left-hand side (negative distances). The longitude at which each profile crosses the axis is noted. Letters on each profile correspond to
those on Fig. 4b, which shows the locations of the profiles.
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Figure 8. (Continued.)
size and distribution of volcanic features varies along the axis and
how the volcanism is related to the tectonics and morphology of the
ridge axis. In particular, we will examine variations in volcanism
between the three magmatic provinces identified by Michael et al.
(2003). A slightly modified version of a numerical algorithm developed by Behn et al. (2004) was used to efficiently and objectively
locate volcanoes. The algorithm searches gridded bathymetry data
for closed concentric contours with a defined maximum perimeter that delineate a region shallower than the surrounding seafloor.
The routine returns the location, height, volume and ellipticity of
each feature. The data used for the analysis was the AMORE swath
bathymetry gridded with 50-m grid spacing. A contour interval of
20 m was used for the analysis. Behn et al. (2004) found that results are insensitive to the choice of contour interval in the range of
10–25 m.
A maximum aspect ratio condition was applied to distinguish volcanic seamounts from elongated topographic highs associated with
faulting. The aspect ratio, α, was estimated for each bathymetric
feature by fitting an ellipse to the basal contour and determining the
principal axes. Only features with α less than a prescribed α max were
identified as volcanic. We found that setting α max = 2.5 generally
served to separate volcanic from tectonic features.
The seamount location algorithm provides information on the
location, size and shape of the individual volcanic features. The
population of seamounts in various regions was characterized by parameters calculated using the maximum likelihood model of Smith
& Jordan (1988). Jordan et al. (1983) found that the distribution
of seamount heights is not Gaussian rather follows an exponential
distribution. Therefore, least-squares is not an appropriate method
to characterize a set of data. Maximum likelihood (Smith & Jordan
1988) is basically a curve fitting technique that is appropriate for
an exponential distribution. It assumes the number, ν, of seamounts
per unit area with height > H is given by
ν(H) = ν0 exp(−βH)
where ν 0 is the average number of seamounts per unit area and
β −1 is defined as the characteristic height of the population. The
maximum likelihood technique formally determines the best fitting
values of ν 0 and β for a set of observed data and the uncertainties
in these estimates.
We tested and calibrated our seamount identification technique
by identifying seamounts on the MAR from 24◦ N to 30◦ N. This
is a well-studied area (e.g. Lin et al. 1990; Sempere et al. 1990)
in which similar analyses have been carried out (Smith & Cann
1990, 1992; Behn et al. 2004). Fig. 10(a) shows the location of
volcanoes identified by our analysis for the central portion (26◦ N–
28◦ N) of the analysed region. Smith & Cann (1990, 1992) limited
their study to seamounts identified on the inner valley floor, defined
as the area between the first major fault scarps (usually greater than
∼200 m) of the two valley walls, to avoid to minimize the effects
of faulting. Our algorithm systematically searches the entire region
and identifies features both on the valley floor (shown as white dots)
and on the valley walls and flanks (shown as yellow dots). Both the
inner floor seamounts and the entire population were analysed to
determine differences between the two data sets.
Fig. 10(b) shows the cumulative distribution of volcanoes that
we identified on the axial valley floor (bottom) and within the entire
studied area marked by the white boxes in Fig. 10(a) (top panels)
for the MAR from 24◦ N to 30◦ N. The solid line on each plot is the
maximum likelihood fit to the distribution of heights. It can be seen
that the data fall very close to an exponential distribution to a height
of about 300 m. Inherent randomness in the height distribution of
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Figure 9. (a) Bathymetry map of the Gakkel Ridge axis from 48◦ E to 58◦ 30 E in the vicinity of the 55◦ E volcanic centre based on AMORE data (Michael
et al. 2003). Location of the area within the EVZ is shown in Fig. 4a. Map is contoured at 100 m intervals and is illuminated from the northwest. White
circles show location of seamounts with height >50 m. Red box outlines the region shown in (b). (b) Detailed bathymetry map of a portion of the Gakkel Rift
valley within the EVZ. Location is shown in (a). Map is contoured at 20 m intervals and is illuminated from the northwest. White circles show the location of
seamounts with height >50 m identified in the region.
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Figure 10. (a) Bathymetry map of the Mid-Atlantic Ridge axis between 26◦ N and 28◦ N. White boxes outline areas searched for seamounts. White dots show
the location of seamounts identified with height >50 m on the rift valley floor and yellow dots show the location of seamounts on the valley walls and flanks.
Map is based on gridded multibeam data where available and bathymetry obtained from satellite data (Smith & Sandwell 1997) elsewhere. Seamount search
utilized only shipboard data. Map is contoured at 250 m intervals. (b) Cumulative count of seamounts plotted against height for the Mid-Atlantic Ridge axis
from 24◦ N to 30◦ N plotted on a linear (left-hand panel) and log (right-hand panel) scale. Solid line shows the maximum likelihood fit to the data. The upper
set of plots shows results for the entire region searched and the lower set of plots shows results for the rift valley floor.
the small number of large volcanoes becomes apparent in the log
plot at greater heights.
In a maximum likelihood analysis, β defines the slope of the
best fitting line in the plot of log(seamount count) versus seamount
height and is thus related to the distribution of volcano heights
(smaller β implies relatively more large volcanoes). As mentioned
above, 1/β, which has units of metres, is defined as the characteristic
height of the distribution. We obtained characteristic heights of 42.1
± 1.6 m for seamounts on the valley floor and 39.8 ± 0.8 m for the
total population of seamounts on the MAR axis from 24◦ N to 30◦ N.
This is essentially the same as the characteristic height of 40 ± 2 m
obtained by Behn et al. (2004) for the same portion of the MAR.
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Figure 10. (Continued.)
The parameter v 0 is the y-intercept of the maximum likelihood fit
to the data (i.e. the total predicted number of seamounts with height
greater than 0 m) divided by the area of the region, and provides
a measure of the density of volcanic features in the area. However,
use of v 0 to characterize the seamount density assumes that there
is a plethora of very small volcanoes lurking below the resolution
of the mapping system. Although we will determine and report
v 0 , we will, instead, primarily use the observed density (observed
number/area) to characterize the abundance of volcanoes in a region.
In doing so, we will count features with relief h > 50 m above the
surrounding seafloor to be consistent with previous studies (e.g.
Batiza et al. 1989; Smith & Cann 1990, 1992, 1993; White et al.
1998; Behn et al. 2004). For the MAR rift valley floor, we identified
399 seamounts with height >50 m and α < 2.5 in an area of
7 192 km2 for an observed density of 55.5 seamounts per 1000 km2 .
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Journal compilation The seamount density for the entire region, including the valley
walls and flanks, is 61.7 seamounts per 1000 km2 (1435 seamounts
in 23 240 km2 ). The 10% higher density for the entire region might
just represent random fluctuation, or it might be the result of identifying tectonic features as seamounts on the valley flanks. Statistical
analysis of the Gakkel Ridge data will be based on seamounts identified on the rift valley floor.
R E S U LT S
Western Volcanic Zone
Fig. 2 shows the location of seamounts with relief >50 m above
the surrounding seafloor identified along the axis of the WVZ. The
analysis identified 76 seamounts on the rift valley floor within an
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Figure 11. Cumulative count of seamounts on the rift valley floor plotted against height for the Western Volcanic Zone on a linear (left-hand panel) and log
(right-hand panel) scale. Solid lines show the maximum likelihood fit to the data. Fig. 2 shows the location of seamounts with height >50 m.
area of 2442 km2 for an observed average density of 31.1 seamounts
per 1000 km2 , which is roughly 55% of the density observed at
the MAR rift valley floor. The characteristic height of the WVZ
seamounts is 29.5 m (Fig. 11). Thus, not only there are significantly
fewer seamounts per unit area observed in the WVZ than at the MAR
but they are also considerably smaller. The smaller size of volcanic
features is evident by the observation that there are no seamounts in
the WVZ taller than 170 m (Fig. 11), whereas 36 seamounts (9.2%
of the total population) are taller than 170 m on the rift valley floor
in the studied portion of the MAR, with seamounts as high as
460 m observed. With a similar height distribution, seven seamounts
taller than 170 m would be expected in the WVZ.
Both the axial morphology and the distribution of volcanoes
are not uniform along the WVZ axis. Morphologically, the four
prominent axial bathymetric highs are located south of about 84◦ N
(Fig. 2), whereas relief along the rift valley floor is lower to the
north of 84◦ N. Fig. 12 shows the seamount density for each of
the computational boxes along the WVZ along with an axial depth
profile. The region north of 83◦ 45 N has a greater concentration
of volcanic seamounts than does the southern portion of the WVZ.
Fig. 13 shows the results of performing separate maximum likelihood analyses of the southern region from 82◦ 57 N to 83◦ 45 N and
the northern region from 83◦ 45 N to 84◦ 33 N. The area south of
82◦ 57 N was not included because it is primarily occupied by the
northern end of the Lena Trough, a deep, obliquely opening and
nearly amagmatic portion of the plate boundary (Snow et al. 2001;
Hellebrand & Snow 2003). The southern region has a seamount
density of 22.6 per 1000 km2 and a characteristic height (1/β) of
26.9 m. The density of volcanoes in this area is 40% of that of the
portion of the MAR that we examined and the individual volcanoes
are much smaller. In the northern region, the seamount density is
52.3 per 1000 m2 with 1/β = 31.4 m. In this portion of the WVZ, the
density of seamounts approaches that of the MAR. However, the individual volcanoes are still much smaller and represent significantly
less erupted magma than volcanoes at the MAR.
Figure 12. Bar graph of the observed density of valley floor seamounts
with height >50 m within each of the computational boxes in the Western
Volcanic Zone. Width of each bar corresponds to the latitude range of the
box as noted on the horizontal axis. Location of boxes is shown in Fig. 2.
Solid line is a profile of the axial depth within the WVZ.
Sparsely Magmatic Zone
The locations of seamounts with h > 50 m from within the SMZ rift
valley are plotted on Fig. 3(a). There are nine seamounts exceeding
50 m in height identified on the valley floor in an area of 1 390
km2 for an observed seamount density of 6.5 per 1000 km2 . The
small number of volcanic features in this region precludes meaningful statistical analysis. It is apparent from Fig. 3(a) that there is a
concentration of volcanic features in the vicinity of the 19◦ E magmatic centre. The observed seamount density in the region of the
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Figure 13. Cumulative count of seamounts on the rift valley floor plotted against height for the Western Volcanic Zone from 82◦ 57 N to 83◦ 45 N (top panel)
and from 83◦ 45 N to 84◦ 33 N (bottom panel). In each area, the data is plotted on a linear (left-hand panel) and log (right-hand panel) scale. Solid lines show
the maximum likelihood fit to the data. Fig. 2 shows the location of seamounts with height >50 m.
magmatic centre is 20.8 per 1000 km2 . The concentration of
seamounts at 19◦ E is similar to that in the southern part of the
WVZ, but the 19◦ E seamounts appear to be somewhat shorter (the
tallest is 80 m high).
If the 19◦ E magmatic centre is excluded, the observed seamount
density drops to 2.7 per 1000 km2 with only three seamounts more
than 50 m high observed in an area of 1101 m2 . All three of these
are east of 26◦ E, in the eastern 25 km of the WMZ as defined
by Michael et al. (2003). In particular, there are no seamounts
taller than 50 m on the rift valley floor for over 185 km to the
southwest of 17◦ 35 E, including the entire SMZ to the west of the
19◦ E magmatic complex. There are 14 smaller features between 20
and 50 m high identified in this region. Some of these may well
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Journal compilation be artefacts in the swath bathymetry (features were not edited out
unless undoubtedly an artefact), but there clearly are some very
small seamounts, particularly near 14◦ E and to the east of 16◦ E
near the 19◦ E complex. These are near the edges, rather than in the
centre, of the valley floor.
Eastern Volcanic Zone
The locations of seamounts with h > 50 m within the EVZ are
plotted in Fig. 4(a). A total of 62 seamounts were identified in an
area of 4 296 km2 , giving a density of 14.4 seamounts per 1000 km2 .
The characteristic height is 22.9 m (Fig. 14). As found throughout
the Gakkel Ridge, individual volcanic structures are significantly
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Figure 14. Cumulative count of seamounts on the rift valley floor plotted against height for the Eastern Volcanic Zone plotted on a linear (left-hand panel)
and log (right-hand panel) scale. Solid lines show the maximum likelihood fit to the data. Fig. 4a shows the location of seamounts with height >w50 m.
DISCUSSION
WVZ is 55% of that at the 24◦ N–30◦ N region of the MAR axis
(Figs 2 and 10a). In addition, volcanic edifices along the WVZ are
significantly shorter than those observed at the MAR (Figs 10b and
11) and thus contain an appreciably smaller volume of erupted lava.
The low melt production is also reflected in the crustal thickness.
Jokat & Schmidt-Aursch (2007) carried out four seismic refraction
experiments in the WVZ and obtained crustal thicknesses ranging
from 1.2 to 4.9 km with a mean of 2.95 km. The two results that
they considered most reliable gave crustal thicknesses of 2.5 and
4.9 km.
Compared with the MAR, the WVZ is therefore actually magmapoor. However it is a ‘genteel’ poverty that is able to ‘keep up appearances’. Although the surface expression of volcanic activity,
and presumably the total melt supply, are drastically curtailed relative to the MAR, the WVZ spreading axis is able to maintain an
MAR morphology with axial volcanic ridges, volcanoes scattered
about the rift valley floor, and rift valley walls consisting of sets
of high-angle normal faults (Fig. 5). Thus, the basic style of melt
generation and distribution, and the associated tectonic activity observed on the MAR, can be maintained even at the lowered melt
production of the WVZ.
Although the western and eastern sections of the surveyed portion
of the Gakkel Ridge are both referred to as ‘volcanic zones’, they
display two very different patterns of magmatism and tectonics.
Eastern Volcanic Zone
smaller than observed at the MAR. The seamount concentration
is significantly greater than in the SMZ, but is less than observed
in the WVZ. There is a clear tendency for volcanic structures to
cluster on the large volcanic centres (Fig. 4a). However, seamounts
are observed along the entire length of the EVZ. As in the SMZ,
most of the seamounts away the volcanic centres are located near
the edge of the rift valley floor (Fig. 4a).
The presence of volcanic features between the large volcanic features is consistent with the observation that SCICEX sidescan data
show bright returns (Fig. 7), suggesting recent volcanism, along the
entire rift valley floor (Cochran et al. 2003). In addition, basalt was
recovered from all but one dredge in the EVZ during the AMORE
expedition, whether the dredge was located on the magmatic centres
or in the areas of deep rift valley between them (Michael et al. 2003),
and seismic refraction data show a thin, low-velocity ‘crustal’ layer
(Jokat & Schmidt-Aursch 2007). Thus, although the rift valley floor
in the SMZ is primarily peridotite, at least a veneer of basalt appears
to be present throughout most of the EVZ.
Western Volcanic Zone
The WVZ has been described as magmatically robust with ‘abundant’ volcanism (e.g. Michael et al. 2003; Baker et al. 2004) due
largely to its morphologic similarity to the MAR (Figs 2, 5 and 10a).
Smith & Cann (1990, 1992, 1993) have shown that the basic extrusive volcanic unit of the MAR is localized, point-source (seamount)
volcanoes, probably fed by dykes injected along the axis (e.g. Smith
& Cann 1999; Magde et al. 2000; Hussenoeder et al. 2002; Dunn
et al. 2005). The number of such structures per unit area at the
The EVZ displays an entirely different form of mid-ocean ridge
tectonics and crustal generation. It is characterized by large volcanic
centres irregularly spaced at 50–150 km intervals along the axis and
separated by stretches of deep (∼5000 m), low relief valley floor
(Figs 4a and 9a). Small volcanic edifices are clustered around the
volcanic centres (Fig. 4a). The volcanic centres also coincide with
large anomalies on aeromagnetic lines, whereas the axial magnetic
anomaly is of lower amplitude and less distinct along the rest of
the rift valley (Brozena et al. 2003; Jokat et al. 2003), supporting
the conclusion based on morphology that volcanism is strongly
focused at the volcanic centres. Although some degree of focusing
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of melt within segments is observed at all spreading rates (e.g. Wang
& Cochran 1993, 1995; Detrick et al. 1995; Magde et al. 2000),
focusing of melt to the volcanic centres appears to be exceptionally
effective at the EVZ, with nearly all of the melt generated beneath
long stretches of the axis transported to the volcanic centres. There
also appears to be little redistribution of melt back along the axis
by crustal level dykes, as observed at the MAR (Magde et al. 2000;
Dunn et al. 2005).
This form of crustal emplacement probably develops when the
lithospheric lid becomes too thick for the melt produced to penetrate, resulting in melt migrating laterally to the volcanic centres.
Cochran et al. (2003) and Dick et al. (2003) both suggested that the
volcanic centres developed as the result of melting of a heterogeneous veined mantle, resulting in local mantle diapirism at the site
of an inhomogeneity. Once established, the volcanic centres will
tend to maintain themselves as low viscosity conduits for melt to
the surface.
Volcanic centres in the EVZ typically extend for 20 to 40 km
along-axis and are oriented perpendicular to the spreading direction.
At the Gakkel Ridge, which is only mildly oblique to the spreading
direction, the volcanic centres are contained within the rift valley
walls and rest on the valley floor (Figs 4a and 9a). At some portions
of the SWIR that are much more oblique to the spreading direction
(e.g. at 55◦ E–56◦ E), volcanic centres with similar dimensions and
gravity signature result in a kinked rift valley with E–W-trending
(perpendicular to the spreading direction) volcanic ridge sections alternating with NE–SW trending deep, non-volcanic sections (Sauter
et al. 2001; Dick et al. 2003).
The SWIR geometry led Dick et al. (2003) to describe the ‘ultraslow’ class of ridge axes as consisting of ‘linked magmatic and
amagmatic accretionary ridge segments’. However, the SWIR volcanic centres are constructed with melt generated beneath the adjacent amagmatic sections and focused to them. Thus, the volcanic
centre and adjacent amagmatic portions of the rift valley form a
single magmatic unit or segment, even if the SWIR axis is partitioned morphologically into spreading-normal volcanic and highly
oblique non-volcanic sections.
The presence of some seamounts in areas of deep rift valley
between the volcanic centres (Figs 4a and 9a) as well as bright
sidescan returns from much of the valley floor (Fig. 7) and the
recovery of basalt in these regions requires that some lava is erupted
away from the volcanic centres in the EVZ. The four seismic lines
reported from the EVZ (Jokat & Schmidt-Aursch 2007) show very
similar crustal thicknesses ranging from 2.6 to 3.3 km with a mean
thickness of 2.9 km. These results overlap with the results from the
WVZ, but have a lower mean thickness, particularly if stations 171
and 172 from the WVZ, which were noisy and considered to be
of poor quality by Jokat & Schmidt-Aursch (2007), are eliminated.
However, given the relatively low density of seamounts between
the volcanic centres (nine seamounts >50 m high are observed in
85 km of rift valley floor between the 43◦ E and 55◦ E complexes and
only one seamount is found in the 45 km between the 55◦ E and 69◦ E
complexes) and their small size (none of those 10 seamounts is over
100 m high and six of them are less than 70 m high), it is unclear
how 3 km of basalt can be built-up. Although basalt certainly covers
the rift valley floor, it may form a relatively thin veneer with some
portion of the low-velocity upper seismic layer made up of altered
mantle rocks, as suggested by Jokat & Schmidt-Aursch (2007) for
the ‘crustal’ layer observed in the SMZ.
The observation that seamounts in the deep regions between
volcanic centres are primarily at the edges of the valley floor and
on the rift valley walls (Figs 4a, 9a and b) suggests that this lava
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ascends along faults. A similar observation on the role of faults
in abetting and determining the location of magmatism was made
at the northern Red Sea by Cochran (2005), who noted that the
small volcanoes in that region are systematically located along faults
bounding large crustal blocks. The northern Red Sea is a latestage continental rift, which has undergone non-magmatic extension
(Bosworth et al. 2005) until recently, when small amounts of magma
have erupted to form the volcanoes observed by Cochran (2005). It
appears that small magma bodies generated beneath a lithospheric
lid may use deep penetrating faults as a conduit to the surface in a
number of tectonic settings.
Sparsely Magmatic Zone
The SMZ is also characterized by intermittent volcanic centres separated by regions of deep rift valley. The distance between volcanic
centres is greater in the SMZ than in the EVZ and there is virtually
no volcanism along the deep valley segments. However, the mode
of melt focusing and crustal generation appears to be the same as at
the EVZ. The extreme focusing of the melt produced in this region
to the few large magmatic centres is demonstrated by the observation that the crustal thickness determined from seismic experiments
in the SMZ range from 1.4–2.5 km with the mean <2 km (Jokat
& Schmidt-Aursch 2007). Jokat & Schmidt-Aursch (2007) suggest
that there may be no magmatic crust in much of this area and that
the low upper velocities that they determined result from serpentinization of peridotite. This is consistent with the lack of ‘bright’
returns, indicative of young volcanic rocks, in sidescan data from
the ridge axis (Fig. 7).
A unique feature of the SMZ is the series of high, axis-parallel
narrow ridges separated by deep troughs that are located on the
southern flank to the west of the 19◦ E volcanic centre (Figs 3b and
8—profiles A–C). These ridges are spaced 10–20 km apart, and
their axis-facing side consists of large-offset scarps dipping toward
the axis at less than 20◦ . These features differ from the low-angle
‘megamullions’ or ‘oceanic core complexes’ observed at the MAR
(e.g. Cann et al. 1997; Blackman et al. 1998; Tucholke et al. 1998)
and SWIR (Cannat et al. 2006). The total offset on these surfaces
is much less than typically observed on oceanic core complexes
and the corrugated surface characteristic of the core complexes is
not observed. These ridges are analogous to the ‘smooth terrain’
described by Cannat et al. (2006) near 64◦ E on the SWIR.
Buck et al. (2005) used numerical models to investigate variations in the nature of mid-ocean ridge faulting as a function of melt
supply as a function of a parameter, M, defined as the fraction of
the plate separation accommodated by dyke injection. They found
that, for a range of M near 0.5, very large offset faults can develop
because the active portion of the fault remains near the axis in the
region of weakest lithosphere. Buck et al. (2005) identify this situation as a mechanism for the creation of oceanic core complexes.
For M = 0.5, the fault moves off-axis into thicker lithosphere and
is then abandoned as a new fault forms near the axis. For large M
(a highly magmatic axis), inward-dipping small-offset, high angle
normal faults are produced (Buck et al. 2005). For small M (an
amagmatic axis), relatively large offset (several kilometres) faults
can still form but are abandoned as they are transported off axis.
In Buck et al.’s (2005) numerical models, a series of small offset
normal faults developed on the opposite side of the ridge from the
large offset faults. This form of periodic formation and abandonment of large-offset faults may be responsible for the characteristic
‘smooth’ terrain observed at places on the Gakkel and Southwest
Indian Ridges.
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Modes of crustal generation
Dick et al. (2003) have argued that the Gakkel Ridge to the east of
3◦ E and portions of the SWIR where similar morphology is found
constitute a separate ‘ultraslow’ class of mid-ocean ridge with morphology, crustal structure and magmatic and tectonic patterns that
are distinct from those found at higher spreading rates. Our analysis
of the number and distribution of seamounts along the Gakkel Ridge
is consistent with this conclusion. Very different patterns of volcanic
activity are seen at the WVZ and at the rest of the Gakkel Ridge.
The WVZ is much less vigorous than the MAR, but maintains an
MAR-like appearance and distribution of magmatic features. It appears that a slow-spreading ‘MAR-like’ form of crustal generation
and tectonics resulting in an axial volcanic ridge, small volcanoes
scattered on the rift valley floor, and rift valley walls made up of sets
of high-angle normal faults can be maintained until the melt supply
decreases and the lithospheric lid increases to some critical value.
At that point, there is an abrupt change to an entirely different form
of ‘EVZ-like’ axial processes that involve much greater focusing
of melt leading to the creation of widely spaced, large magmatic
centres separated by areas where there is at best a thin veneer of
basalt overlying mantle rocks with volcanoes mainly located on the
rift valley walls and fed by melt ascending along faults.
Dick et al. (2003) suggest that the transition between the ‘slow’
and ‘ultra-slow’ modes of crustal generation occurs at about 12 mm
yr−1 . They base this on observations at the Gakkel Ridge, where the
abrupt transition between the WVZ and the SMZ near 3◦ E occurs
at a spreading rate of 12.2 mm yr−1 , and the westernmost portion
of the SWIR near 16◦ E where the transition occurs at an effective
spreading rate (component of plate separation perpendicular to the
ridge trend) of about 12.4 mm yr−1 .
However, as noted in passing by Dick et al. (2003), the transition cannot be dependent simply on effective spreading rate. The
influence of factors other than spreading rate can be seen at the 3◦ E
transition from ‘slow’ to ‘ultra-slow’ morphology on the Gakkel
Ridge. An indication that the threshold transition from ‘slow’ to
‘ultraslow’ form of axial processes can be caused and localized by
factors other than simply spreading rate comes from variation in
magmatic activity approaching the 3◦ E WVZ–SMZ boundary. The
number and size of volcanic seamounts in the WVZ does not decrease from southwest to northeast approaching the EVZ. Instead,
both parameters increase significantly. This is consistent with seismic refraction data that gives a crustal thickness of 2.5 km profile
190 centred near 83◦ 30 N and 4.9 km for Profile 180 centred just
north of 84◦ N (Jokat & Schmidt-Aursch 2007). Thus, it would appear that the ‘vigour’ of the magmatic system in the WVZ increases
rather than decreases, approaching the 3◦ E boundary with the SMZ.
Similarly, in the SMZ, magmatism does not increase approaching
the WVZ. The ridge axis from 3◦ E to 17◦ 35 E, is the most amagmatic section of axis on the known portion of the Gakkel Ridge,
and perhaps in the entire mid-ocean ridge system, with no volcanic
features >50 m high observed for the entire 185 km and extensive
areas of exposed peridotite. The abrupt transition at 3◦ E is thus from
the most magmatic to the most amagmatic sections of the Gakkel
Ridge.
Goldstein et al. (2008) identified an abrupt isotopic boundary
between 13◦ E and 16◦ E in the centre of the SMZ. Lavas recovered
to the west of the boundary, including in the WVZ, have isotopic
characteristics typical of the ‘DUPAL’ isotopic anomaly of Indian
and South Atlantic MORB (Dupre & Allegre 1983; Hart 1984)
while those from east of the boundary are consistent with normal
‘Pacific–Atlantic’ isotope trends. Goldstein et al. (2008) drew an
analogy between the isotopic boundary on the Gakkel Ridge and the
Australian–Antarctic Discontinuity (AAD) on the Southeast Indian
Ridge (SEIR), which is also the site of an abrupt boundary between
DUPAL and Pacific mantle provinces (Klein et al. 1988, 1991; Pyle
et al. 1992). The AAD is a very deep area of chaotic morphology
(Weissel & Hayes 1974; Palmer et al. 1993) characterized by low
melt production and thin crust (Anderson et al. 1980; Forsyth et al.
1987; Kuo 1993; Tolstoy et al. 1995). At both the Gakkel Ridge and
the SWIR, a sharp, distinct compositional boundary in the mantle
occurs within a region where the ridge axis is deep and extremely
amagmatic. In addition, the portion of the SEIR immediately to the
east of the AAD is one of the most vigorous sections of the SEIR,
with a shallow crustal depth and a well-developed ‘EPR-like’ axial
high (West et al. 1994). In both of these cases, a sharp boundary
between magmatic and extremely amagmatic portions of the ridge
axis coincides with a major chemical boundary that occurs within
the amagmatic region.
Another example of the influence of parameters other than
spreading rate, and in particular discontinuities in mantle composition, in localizing the transition from ‘slow’ to ‘ultra-slow’
spreading comes from considering two portions of the SEIR on either side of the Melville transform at 60◦ 45 E. The eastern SWIR,
in the vicinity of the Melville transform, is spreading almost exactly
N–S at a fairly constant rate of ∼14 mm yr−1 (Patriat & Segoufin
1988; DeMets et al. 1994; Cannat et al. 2003).
The SWIR to the east of the Melville transform is characterized
by widely spaced large volcanic centres separated by deep, nearly
flat stretches of rift valley (Mendel & Sauter 1997; Mendel et al.
2003; Sauter et al. 2004; Gomez et al. 2006) and appears analogous
to the EVZ or SMZ of the Gakkel Ridge. Mendel & Sauter (1997)
determined the location of seamounts along this portion of the
SWIR and found that they are extremely sparse away from the large
volcanic centres A deep-tow sidescan survey found no evidence
of recent volcanism in an 82 km-long section between 64◦ 31 E
and 65◦ 20 E (Sauter et al. 2004) in an area which trends nearly
orthogonal to the spreading direction with an effective spreading
rate of 13.2–14 mm yr−1 . In contrast, the area immediately to the
west of the Melville Transform is associated with elongated volcanic
ridges and the sidescan survey found that segments in this area are
covered with volcanic terrain, most of it relatively fresh (Sauter et al.
2002; Gomez et al. 2006). Mendel & Sauter (1997) report a much
higher and more evenly distributed population of seamounts in this
region than to the east of the transform. This area, which appears
similar to the WVZ on the Gakkel Ridge, has a trend of about N50◦ E
and is thus about 40◦ from perpendicular to the spreading direction
with an effective spreading rate of ∼10.7 mm yr−1 .
Thus, at the eastern SWIR, segments with an effective spreading rate <11 mm yr−1 have a more vigorous ‘slow’ or WVZlike morphology and segments with an effective spreading rate
>13 mm yr−1 have an amagmatic EVZ- or SMZ-like ultraslow
morphology. Clearly, effective spreading rate is not the dominant
factor controlling the form of axial tectonics at this area. As was the
case for the transition from ‘slow’ to ‘superslow’ spreading at 3◦ E
on the Gakkel Ridge, the Melville transform, separating these two
areas on the SWIR is the location of a significant discontinuity in
mantle composition (Meyzen et al. 2003, 2005).
C O N C LU S I O N S
A quantitative characterization of seamount volcanism along the
axis of the Gakkel Ridge shows both that volcanism at the Gakkel
Ridge is much reduced compared with the MAR and that there
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are significant variations in the extent and distribution of volcanic
features in different sections of the Gakkel Ridge. The density of
constructional volcanic structures in the WVZ is 55% of that determined using the same techniques at the MAR and the characteristic
height and the volume of seamounts at the WVZ axis is significantly
less than at the MAR. These results imply a greatly reduced melt
supply to the WVZ. These results are consistent with seismic results
that suggest that the crustal thickness at the WVZ is approximately
half that normally found at the MAR (Jokat & Schmidt-Aursch
2007). However, in spite of the severely reduced melt supply, the
WVZ still demonstrates the basic pattern of MAR morphology
with axial volcanic ridges, volcanoes scattered across the rift valley
floor, and rift valley walls consisting of sets of high-angle faults.
It thus appears that the ‘slow spreading’ form of crustal accretion
characterized by partial focusing of melt to the centre of segments,
redistribution of magma along-axis by dykes, and high-angle normal faulting (e.g. Smith & Cann 1993; Cannat 1996; Hooft et al.
2000; Magde et al. 2000; Dunn et al. 2005) can be maintained at
the greatly reduced melt supply of the WVZ.
The Eastern Volcanic Zone displays a completely different mode
of crustal accretion. The density of volcanic features is about 45% of
that found at the WVZ (and about 25% of the density at the MAR).
Seamounts cluster near the large magmatic centres but are observed
along the entire length of the EVZ. Away from the magmatic centres,
seamounts tend to be located at the edge of the rift valley or on
the valley walls. The Sparsely Magmatic Zone displays the same
pattern of volcanism as the WVZ but taken to an extreme. There are
no seamounts on the valley floor for 185 km to the west and 45 km
to the east of the 19◦ E volcanic centre. It appears that melt produced
by melting of mantle upwelling in response to plate separation in
the EVZ and SMZ is efficiently focused to widely spaced magmatic
centres. Magma erupted between the magmatic centres reaches the
surface along deep-cutting faults with the result that seamounts are
occasionally found on the valley walls but rarely on the valley floor.
The transition between the two forms of crustal accretion is a
threshold step between the two completely different modes that
appears to occur when the cold lithospheric lid reaches a critical
thickness. Spreading rate is one of several factors that affect which
style of seafloor spreading occurs. On the Gakkel Ridge, the transition from the WVZ to the SMZ is localized by a major compositional
boundary in the mantle (Goldstein et al. 2008).
AC K N OW L E D G M E N T S
I thank Mark Behn for providing his seamount location algorithm
and for assistance in implementing it, Margo Edwards of the University of Hawaii and Hans-Werner Schenke of the Alfred Wegner
Institute played critical roles in the processing of the bathymetry
data sets used in this study. D. K. Smith and W. Jokat provided thorough and thoughtful reviews that greatly improved the manuscript.
The GMT software package (Wessel & Smith 1998) was used extensively for data analysis and the preparation of figures. This work
was funded by NSF grant OPP-0352642. LDEO contribution 7167.
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